The irradiation of metals and semiconductor materials by an ion beam provides a means for effecting the doping of such materials in a controlled and rapid manner. Ion implantation, as the process is known, is accomplished by irradiating, for example, a semiconductor wafer with an ion beam of controlled intensity for such integrated exposure as needed to provide the desired dopant or impurity concentration. See U.S. Pat. Nos. 4,283,631 and 4,383,180, issued to N. Turner for descriptions of typical ion implantation apparatus.
It is well known that ion beams of twice the final energy for a given acceleration voltage can be obtained in many ion-implanter machines by using doubly-charged ions. Doubly-charged ions can be provided by controlling the analyzer magnetic field in ion implantation apparatus in a similar manner as one provides singly-charged ions. However, a doubly-charged ion beam can be contaminated with lower energy ions which are singly-charged. This form of contamination is prevalent in ion implanters which analyze the mass of the ion beam in the analyzer magnetic field at a relatively low voltage, usually in the order of 25 to 35 kilo-electron-volts (keV), prior to the final acceleration of the desired doubly-charged ions to higher voltages. During this phase of operation of the ion implanter, the doubly-charged ions are contaminated in any one or more of the following ways. First, contamination can occur from the dissociation of molecular ions, for example, phosphorus molecular ions (P.sub.2.sup.+) and arsenic molecular ions (As.sub.2.sup.+), in the region between the source of the ions and the analyzer magnet. As known, the analyzing magnet is a mass momentum analyzer and operates on the mass energy of a particular specie. For example, when a phosphorus molecular ion (P.sub.2.sup.+) at 25 keV dissociates into a singly-charged phosphorus ion (P.sup.+) and neutral phosphorus atoms (P.sup.0) with energy at 12.5 keV each, the singly-charged phosphorus ion (P.sup.+) at 12.5 keV cannot be separated from the doubly-charged phosphorus ions (P.sup.++) at 25 keV by the analyzer magnet since both the ions have the same energy per unit charge.
Second, the beam can be contaminated by those doubly-charged P.sup.++ ions which have picked up secondary electrons in the acceleration column of the ion implantation apparatus to form singly-charged ions. The secondary electrons are scattered in the direction of the ion beam. Thus, the electrons (e) attracted in the doubly-charged phosphorus ions (P.sup.++) result in a singly-charged phosphorus ion (P.sup.+). This event can be represented as P.sup.++ +e.fwdarw.P.sup.+.
Third, the doubly-charged P.sup.++ ions attract electrons by means of collisions with residual gas molecules (X) in the acceleration column. This process can be represented as EQU P.sup.++ +X.fwdarw.P.sup.+ +X.sup.+
where X denotes an unknown particle or atom, such as an oxygen or nitrogen atom.
Accordingly, it is clear there is a need to reduce the contamination of doubly-charged ion beam by singly-charged ions in ion implantation processes.